FIG. 1 illustrates a two-mirror telescope 20 as in the prior art with both mirrors in alignment, and is presented as an overview. The telescope 20 comprises a primary reflector 22 that defines an active concave surface 24 that is typically parabolic. Facing the active concave surface 24 is an active surface 30 of a secondary mirror 28 that is typically hyperbolic. Though the active surface 30 of the secondary mirror is depicted as convex, it may alternatively be concave. As used herein, the term active surface refers to an operational surface of the telescope 20. Disposed between the active convex surface 30 and the active concave surface 24 at the focus of the mirror system is a target 34, such as a radiation sensor. Each of the primary reflector 22, the secondary reflector 28, and the target 34 are aligned on an optical axis 36, preferably centered on the axis 36. When aligned, the optical axes 36 of the primary 22 and secondary 28 reflectors are co-incident. The target 34 is preferably centered on the optical axis 36, and may define planar or arcuate active surfaces.
By convention used consistently throughout this disclosure, the optical axis 36 is parallel to a z-axis, and the x-y plane is perpendicular to the optical axis 36. As depicted in FIG. 1 and consistent throughout this disclosure, the x-axis is vertical and parallel to the pages on which the figures are drawn, and the y-axis is perpendicular to the plane defined by the pages on which the figures are drawn. Rotation of a reflector 22, 28 about the y-axis is referred to as tip, and rotation about the x-axis is referred to as tilt. Typically, rotation about the z-axis does not affect telescope alignment as the reflectors 22, 28 are usually symmetric about the z-axis.
When in alignment as shown in FIG. 1, incident light along pathways parallel to the optical axis 36 (the z-axis) strikes the active concave surface 24 of the primary mirror 22 and are reflected toward the active convex surface 30 of the secondary mirror 28, where they are reflected again toward the target 34. Thus, incident light incident on the larger area of the active concave surface 24 is concentrated in a relatively small area of the target 34 (or a portion of the target 34) for better resolution. Assuming both reflectors 22, 28 define the same optical axis 36, the two mirrors define five degrees of freedom: tip and tilt for each of the primary and secondary reflectors 22, 28; and spacing of the reflectors 22, 28 relative to one another in the z-direction. Tipping either of the reflectors 22, 28 causes the reflected incident light to move in the x-direction. Tilting either of the reflectors 22, 28 causes the reflected incident light to move in the y-direction. Moving the reflectors 22, 28 relative to one another along the z-direction causes the focal point of the incident light beams to move in the z-direction.
The two mirrors were typically aligned in the prior art using mechanical means that moved the mirrors in a multi-step process to achieve alignment in all five degrees of freedom sequentially. Typically, each degree of freedom was set manually through trial and error and required much expertise to be done efficiently. Prior art methods were limited in that alignment of only one or two degrees of freedom could be checked at once. The problem with the prior art is that an interferometer allows a user to see the “quality” of the alignment from the system test interferogram in real time, but does not isolate which parameter is to be changed to get the required performance. Therefore, any improper alignment required iteratively checking all degrees of freedom to identify the mis-aligned mirrors. What is needed in the art is an apparatus and method to view all five degrees of freedom, and to isolate one or more mis-aligned axes and reflectors when greater precision and better alignment is desired.